Q: Engine stalls during acceleration, around 60 mph

I purchased the car a month ago and olnly done a dozen trips or so. In about 1 in 3 trips the engine stalls if I try to go over 60mph. It behaves as though I take foot off gas, no matter if I'm still pushing the pedal or even trying to go harder on it. Rpm only picks up slightly, then falls 500-600 and car slows to 50-55mph, then I can push gas again and easily pick up to 60-62mph, then it loses power again. Seems to happen in really wet weather. Then I let car sit for 2 hours and it drives great the next 50 miles, easily picking 75mph. I took it to mechanic after purchase to change spark plugs, he said everything else looks good. Neither he nor i didn't find any oil leak, even though it was 1 quart low at oil change, 4000 miles since last oil change done by old owner. Has minor exhaust leak but mechanic said it won't hurt anything. Didn't notice the problem with acceleration before we changed spark plugs, but I didn't do long trips before that.

My car has 170000 miles.
My car has an automatic transmission.